Support Reactions Sign Convention

[Anonymous]

I am working with the geotechnical engineer to decide the foundation type for my bridge. So we connect all the bridge super structure to an arbitrary fixed support. We then take the forces from the fixed support as an input for the geotechnical assessment.

I am finding it strange that the Moment in Y direction for both Abutment 1 & 2 have the same sign convention although the rotation is in the opposite direction.

Will appreciate any help to understand the sign convention

[dave_geeves]

Hi ,

I agree that this does not seem to be what I would expect, but it is difficult for me to comment without having the model to investigate.  For example, I don't know whether the supports are defined relative to a local coordinate system or whether they are just global supports.  Could you please zip up your model files and attach them to a reply so that I can investigate.  Thanks.

Kind regards

Dave Geeves

[Anonymous]

Thanks Dave for the reply.

I am after the moment reactions in Node 177 and 176. Other supports are springs to represent the soil on my semi integral bridge.

Best Regards,

Ammar

[dave_geeves · Accepted solution]

Hi Ammar,

Thanks for attaching your structure file which I have successfully loaded and run and I can replicate the behaviour you describe.  However I had difficulty in understanding where the moments are coming from as the supported node resisting My moment is only connected to a beam (Sill Beam) which has torsional moments released at the connections to the main girders. 

So in essence the bridge deck is acting as simply supported as there is no torsional resistance from the transverse members at the supported node. 

There is however continuity in local flexural bending of the sill beams and because the deck is skewed these moments will be resolved into global directions at the supported node where there will be a global My component that will provide an MY support reaction.  As this flexural skewed transverse moment is the same sign at each end of the deck the resolved global MY support reaction will also be of the same sign.

I'm not really sure what you are trying to achieve here by releasing the torsion in the sill beam so I can't really help any more at this point

I hope this has explained the results you are getting.  If this has answered your query then please mark my reply as a solution so that others may benefit.  Thanks.

Kind regards

Dave Geeves